This is a test implementation in maya of the soft IK explained by Andy Nicholas, in http://www.xsi-blog.com/?p=108. It is implemented as a maya constraint, created by a maya python plugin. It's intended is is to set things up so that the ik handle constrained to the animation control...

This code is totally unsupported and provided as is. It wasn't even really intended for public consumption - it was a small test I made on my own that I hosted on github as an easy way to coordinate work at home and at school.

However, it seems that a few people have stumbled across this and asked how to use it, so I decided to provide a basic example showing how to use it. If you don't know basic scripting, or can't follow the example, then move on... this is NOT a complete end-user package. (There's no command to create/setup a SoftIk constraint... though you could certainly make one using the provided example as a guide...)

If you still want to try it out, though, you'll need to install this script somewhere in your Maya plugin path. Look at the official maya docs if you need help doing this.

Then, try running this code in a python script editor:

import pymel.core as pm
if not pm.pluginInfo('pmSoftIK.py', q=1, loaded=1):
    pm.loadPlugin('pmSoftIK.py')

pm.newFile(f=1)

j1 = pm.joint(p=(0, 0, 0))
j2 = pm.joint(p=(4, 0, 0))
j3 = pm.joint(p=(4, 0, 4))
ikHandle = pm.ikHandle(sj=j1, ee=j3)[0]
handleParent = pm.group()
cube = pm.polyCube()[0]
cube.translate.set(4,0,4)

softConstraint = pm.createNode('pmSoftIkConstraint')


# Set the joint-chain-length
# This bit is a bit of hack... really, I should have a command that computes
# this distance, or have the node itself dynamically figure it out.  I don't
# though. You'll have to figure out the total joint chain length yourself...
softConstraint.chainLength.set(8)


# Now make all the required constraint connections..
j1.worldMatrix[0].connect(softConstraint.startJointWorldMatrix)
cube.worldMatrix[0].connect(softConstraint.targetWorldMatrix)
ikHandle.parentInverseMatrix[0].connect(softConstraint.constraintParentInverseMatrix)
softConstraint.constraintTranslateX.connect(ikHandle.translateX)
softConstraint.constraintTranslateY.connect(ikHandle.translateY)
softConstraint.constraintTranslateZ.connect(ikHandle.translateZ)

# ...and, for convenience, and an attribute onto our cube control for
# playing with the softRatio
cube.addAttr('softIkRatio', keyable=True, hidden=False)
cube.softIkRatio.connect(softConstraint.softRatio)
cube.softIkRatio.set(.2)

Now, just move the cube around, and see how the ik handle / joint chain responds. Also, try changing the softIkRatio on the cube to see how it affects the behavior...